(1) Field of the Invention
The present invention relates to a system for measuring the position of a terminal through cellular communication.
(2) Description of the Related Art
Japan laid-open Patent 7-181242 has disclosed a technology for measuring the position of a terminal by observing the timing of receiving pilot signals from base stations accommodated by a cellular communication system having the code division multiple access (CDMA) method implemented therein. The base stations operate synchronously with satellites included in the global positioning system (GPS).
FIG. 2 and FIG. 3 are concerned with the technology described in the Japan laid-open Patent 7-181242.
FIG. 2 shows a terminal 24 for receiving reference signals 25, 26, and 27 sent from base stations 21, 22, and 23 synchronized with a satellite 20 included in the GPS. FIG. 3 shows an example of the results of correlation.
The terminal 24 correlates the received signals with the patterns of specific codes so as to detect the timings of receiving the reference signals sent from the base stations.
According to the CDMA method, common pilot signals sent from the base stations have specific patterns. The terminal correlates the received signals with the patterns of the pilot signals so as to detect the reception timings.
Each of the base stations adjusts a reference time i by the GPS synchronization and transmits a pilot signal according to a specific transmission timing determined according to the adjusted system time. The specific transmission timing corresponds to an offset time. Information of the offset time is transmitted on a sync channel, and a terminal can acquire the information freely. The terminal calculates a difference between a measured reception timing and the known transmission timing so as to work out a delay time or a time required for propagation of a radio wave.
FIG. 3 shows examples of the results of correlation. The graphs of FIG. 3 are referred to as delay profiles indicating a path along which a signal is propagated with a delay.
The axis of abscesses indicates the reception timing corrected based on the transmission timing, that is, the delay time. The unit of delay time corresponds to each of chips in a spreading code. The axis of ordinates indicates the output or results of correlation operation. A larger correlation value indicates that a signal is received with an indicated delay time spent, that is, a path is determined.
According to the results of correlation, a relative delay time required for a radio wave to reach a terminal after transmitted from a base station can be calculated. The delay time is a relative value because a terminal is unaware of an absolute time instant. A product of an obtained relative delay time by a light velocity indicates a difference between relative propagated distances. Once the difference between relative propagated distances is calculated as to at least three base stations, the position of the terminal can be estimated according to the triangulation method.
Referring to FIG. 4, a method of selecting base stations will be described below. How to select base stations has a significant meaning in measuring a position through cellular communication.
In cellular communication, base stations and a terminal are located on the same plane. Precision in a measured position varies depending on the positional relationships among the base stations and terminal.
FIG. 4 shows the distribution of errors of a measured position when the position to be measured is shifted as 90, 91, and 92 relative to base stations 50, 51, and 52.
Here, it is assumed that a signal transmitted from each of the base stations includes an estimation error of a delay time having a uniform distribution in a range of xc2x10.2xcexc sec or less. A black area indicates the spread of errors of each measured position deriving from the intentionally appended error of the delay time. Although the intentionally appended error of the delay time is the same, the spread of errors of the measured position varies depending on the positional relationships among the terminal and base stations. This means that the precision in the estimated position of a terminal varies depending on selected base stations.
An object of the present invention is to improve precision in estimation.
According to the present invention, the triangulation method is used to calculate a plurality of estimated positions relative to various combinations of three base stations selected from among three or more base stations. The estimated positions are averaged in order to reduce an error of an estimated position.
According to the triangulation method, as shown in FIG. 5, if three unit vectors originated from a terminal towards three selected base stations are added up, an error of an estimated position becomes minimal when the length of the resultant composite vector is zero. As the length of the composite vector gets longer, precision in an estimated position of a terminal deteriorates. As long as the length of the composite vector falls below a certain threshold value, an estimated position contains a small error. As an easy judgment, it may be judged whether a terminal lies within an estimated triangle, that is, whether the length of the composite vector is equal to or smaller than 1, in order to make the judgment for a short period of time by employing a digital computing apparatus.